The herd massed together on Hardangervidda, the mountain plateau in central Norway, and on Tuesday afternoon had moved within three kilometres of the country's Highway 7.
"This is probably the first time the road has been closed for such reasons," the NPRA's local section head Lars Øyre said in a press statement. "Several times, the animals have got close to the road, and we've been on the point of pressing the button to shut the road down, but they've always turned away in time.".
He said he could not predict when the road would reopen.
"It depends on the animals," he said.